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Visible Light Communication: a System Perspective—Overview and Challenges

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Visible Light Communication: a System Perspective—Overview and Challenges sensors Review Visible Light Communication: A System Perspective—Overview and Challenges Saeed Ur Rehman 1,*, Shakir Ullah 1, Peter Han Joo Chong 1, Sira Yongchareon 2 and Dan Komosny 3 1 Department of Electrical and Electronic Engineering, Auckland University of Technology, Auckland 1010, New Zealand; [email protected] (S.U.); [email protected] (P.H.J.C.) 2 Department of Information Technology and Software Engineering, Auckland University of Technology, Auckland 1010, New Zealand; [email protected] 3 Department of Telecommunications, Brno University of Technology, Technicka 12, 601 90 Brno, Czech Republic; [email protected] * Correspondence: [email protected] Received: 10 January 2019; Accepted: 1 March 2019; Published: 7 March 2019 Abstract: Visible light communication (VLC) is a new paradigm that could revolutionise the future of wireless communication. In VLC, information is transmitted through modulating the visible light spectrum (400–700 nm) that is used for illumination. Analytical and experimental work has shown the potential of VLC to provide high-speed data communication with the added advantage of improved energy efficiency and communication security/privacy. VLC is still in the early phase of research. There are fewer review articles published on this topic mostly addressing the physical layer research. Unlike other reviews, this article gives a system prespective of VLC along with the survey on existing literature and potential challenges toward the implementation and integration of VLC. Keywords: visible light communication; optical communication; LED communication; VLC networks 1. Introduction In the 1980s, the development of high-efficiency red, orange and yellow light emitting diodes (LEDs) have fueled the idea of replacing the solid-state lighting for illumination purpose. It was until 1996 when the first white LED was commercially introduced in the market for sale [1]. LED lights are highly powered efficient, low carbon emissions, free from mercury, durable and produce good quality illumination. LED lights have 75% less power consumption and last 25% longer than traditional incandescent lamps [2]. With the decreasing prices and low power consumption of LED’s, it is estimated that the market share of LED lighting would increase to 69% in 2020 [1]. The exponential growth of data in the last two decades has raised concerns over the electricity consumption of information communication technology (ICT) infrastructure. It was estimated that ICT infrastructure accounted for 4.6% of worldwide electricity consumption in 2012 and projected to increase in the future despite emphasising on the introduction of power efficient technologies [3,4]. By 2030, the contribution of the ICT in greenhouse release would increase up to 23%, and at the worst, it can go up to 50% [5]. The future of the internet of everything (IoE) connecting people, processes, things, data and everything would require internet connectivity at all times. IoE would further increase the deployment of ICT infrastructure, thus increasing the power consumption. Apart from providing illumination at low-cost, LED lights have been used in several other applications, e.g., indoor farming and plantation [6,7], medical applications [8,9]. The easy availability of LED lights at home, offices and public spaces make it an affordable candidate to deal with the radio frequency (RF) spectrum scarcity as well as providing an energy efficient communication system. It is envisioned that existing lighting infrastructure should provide illumination as well as data connectivity. Sensors 2018, 19, 1153; doi:10.3390/s19051153 www.mdpi.com/journal/sensors Sensors 2018, 19, 1153 2 of 22 Visible light communication (VLC) is a new paradigm that could revolutionise the future of wireless communication. In VLC, information is transmitted by modulating the visible light spectrum (400–700 nm) that is used for illumination. The information signal is superimposed on the LED light without introducing any flickering to the end user. Thus, it would be “green” as compared to providing two separate sources for illumination and communication network connectivity. On the other hand, the exhaustion of low-frequency bands to cope with the exponential growth for the highspeed wireless access is another reason for exploring new technologies. The visible light spectrum is unlicensed and hardware readily available, which can be used for data transmission. Furthermore, the exponential improvement in the high power light emitting diodes is an enabler for high data rate VLC Network. It has the potential to provide high-speed data communication with improved energy efficiency along with security/privacy. Standardisation efforts such as visible light communications association (VLCA) standards and IEEE 802.15.7 shows that VLC would augment existing wireless networks in coming years. VLC can have applications in indoor wireless communication [10], intelligent transport system [11,12], smart cities [13], localisation in warehouses/robotics [14–16], human sensing [17], safe and hazard-free data access in hospitals [18], toys and theme parks [19], indoor point to point (PPP) communication and vehicular communication [12]. VLC in its basic form like any other communication system in the downlink consists of a LED as a transmitter, a free space optical communication channel and a photodetector or an image sensor as a receiver. The uplink could be a WiFi transmitter or an IR transmitter or LED based VLC transmitter. Much of the work is focused on the downlink transmission to increase the data rate and improve VLC performance under different environmental conditions such as shadowing, non-LOS scenario and mobility. However, the uplink is equally essential for seamless integration of VLC in the existing ICT infrastructure. Unlike other review articles, this paper provides an overview of the VLC from the uplink and system perspective. This review article critically analyses the existing solutions of the VLC network from the system as well as uplink perspective. This paper has following contributions • We have critically analysed the existing literature of the VLC regarding the uplink from the user device to VLC access point (VAP). • We have discussed the open challenges associated with the use of existing RF spectrum for uplink connectivity. The rest of the paper is organised as follow: Section2 discusses the history and standardisation efforts. Section3 discusses applications of VLC. Section4 gives an overview of the different technologies used in the uplink and discusses its limitation. Section5 summarises open research challenges toward the integration of VLC in the existing systems. Section6 concludes the paper. 2. Visible Light Communication System In the early 1990’s, mobile phones were mainly used for voice conversation or text messaging. However, the introduction of the iPhone in 2007 has started a new era in wireless communication [20]. Nowadays smartphones are equipped with all kinds of sensors and applications to provide health monitoring, video chatting, online streaming along with bank transactions and using cloud services. A larger bandwidth should be allocated for wireless communication in order to provide seamless connectivity and higher data rates. However, frequency spectrum below 5 GHz is well utilised, which leave no room to relocate spectrum for mobile communication. This led scientists to seek new wireless technologies that can fulfil the needs of the higher data rate at a low cost. One such candidate is the use of the visible light spectrum. It has the advantage of its availability (LED lights), link level security, higher bandwidth, and frequency reuse. Furthermore, the demand for data is higher in an indoor environments because 80% of the time people stay in an indoor environments [21]. Thus using the VLC in indoor applications for high data rate applications would free up the precious RF spectrum for other future applications such as autonomous cars and smart cities. Sensors 2018, 19, 1153 3 of 22 The history of VLC goes back to Romans when polish metallic plates were used to reflect sunlight and convey signals over a long distance. In 1794, Claude Chappe developed a semaphore system consisting of a series of towers equipped with mounted arms to transmit information. In the late 19th and early 20th Century, heliograph was used for long distance communication. In heliograph, the sunlight was reflected with a mirror to transmit Morse code. The British and Australian armies used it till 1960. Graham Bell is mostly known for his invention of the modern telephone, which uses electricity to transmit voice. However, Bell described the photo-phone as one of his important inventions [22]. Photophone uses voice to vibrate the mirror, which in turn is used to modulate the sunlight. It was the idea of Graham Bell, which led to fibre optic communication. The first commercial fibre optic communication system was deployed in 1975 and was capable of operating at a bit rate of 45 Mbit/s. VLC is a form of optical communication that instead of using a guided media (fibre optic) operates in the open air in close proximity of two to three meters. In 2003, the term VLC was coined first by Nakagawa Laboratory at Keio University, Japan [23]. The Nakagawa Laboratory demonstrates the first VLC system at Keio University in 2000. Light emitting diodes (LEDs) are used for transmitting the data. Liang et al. have proposed a VLC system consisting of red-green-blue (RGB) LEDs as a transmitter and complimentary metal oxide semiconductor (CMOS) image sensor as a receiver [24]. The authors observe that RGB LEDs can increase the VLC transmission data rate with wavelength division multiplexing (WDM). For such systems, colour signals are isolated using colour filter array which relies on colour-sensitive sensing elements. However, the wide optical bandwidth of colour filters can increase spectral overlap between channels and can cause inter-channel interference (ICI). In order to solve this issue, the authors have applied CMOS image sensors with multiple input–multiple output (MIMO) to mitigate the ICI and demodulate the rolling shutter pattern.
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